1. Field of the Invention
This invention relates generally to an improvement in the curing of advanced composites in an autoclave and more particularly, but not by way of limitation, to a novel method and apparatus for the ultrasonic monitoring of an advanced composite during the cure cycle within an autoclave.
2. Description of the Prior Art
In the manufacture of advanced composites it is common to position the advanced composite part in an uncured state on a tool such as a bond jig for positioning within an autoclave for curing under pressure and at elevated temperatures. An advances composite may be considered to be a fiber resin matrix that has been formed by hand lay up, filament winding, tape laying or other means. The fiber resin matrix may be formed upon the tool upon which it is to be cured or it may be formed elsewhere and transferred to the tool for curing as in the instance of filament winding. The fiber resin matrix may also be formed by alternating layers of sheet resin with layers of dry fiber that have woven into a fabric or the layers of the woven fiber may be preimpregnated with the resin in an uncured state (prepreg).
In any event the fiber resin matrix, graphite/epoxy being a common one, is positioned on the tool and then covered with a bag made from a flexible plastic material which is sealed to the upper surface of the tool which may be configured in a desired manner by a suitable sealing tape. The bag may be penetrated by instrumentation and a means for drawing a vacuum within the bag so that it closely surrounds the part which may also be covered with various sheets such as breather sheets.
When the tool and the bagged composite part are placed in an autoclave for curing, the composite part is then subjected to a predetermined regimen of pressure and temperature in what is known as a cure cycle. During the cure cycle of the composite part the viscosity of the composite part will vary before it finally hardens into its final shape. If it were possible to determine with sufficient specificity the viscosity of the composite part it would be possible to optimize the cure cycle and to also adjust the cure cycle while the part is being cured to achieve the optimal curing of that particular composite part. This is the general objective of the present invention.
The following patents, which of interest in the general field to which the invention pertain, do not disclose the particular aspects of the invention that are of significant interest.
U.S. Pat. No. 4,478,072 issued on Oct. 23, 1984 to Harold T. Brown discloses an apparatus for determining the concentration of finely divided coal particles dispersed in a fuel oil medium. The apparatus comprises in combination (a) an ultrasonic pulser, (b) an ultrasonic receiver, (c) a gated peak detector, (d) signal conditioning and display circuits and (e) an ultrasonic transducer assembly including a layer of silicone grease interposed between the sample of fuel oil having the coal particles dispersed therein. The pulse generated by the pulser is reflected and the peak amplitude of the reflected pulse is indicative of the solids content of the dispersion. There is no attempt at constant monitoring of a curing composite by monitoring of the viscosity of the composite as is taught by the instant invention.
U.S. Pat. No. 4,574,637 issued on Mar. 11, 1986 to Laszlo Adler et al discloses a method for measuring surface and near surface properties of materials. The properties of the surface layer of a material are measured by a technique which employs the transmission of ultrasonic waves from varying angles of incidence into a specimen, from a transducer at a point spaced from the specimen. The backscattered waves are detected and evaluated from the varying angles to detect the local maximum intensity, from which the corresponding properties of the material are determined. The Adler method induces Raleigh surface waves by varying the incidence of impinging ultrasonic waves. The properties of the surface material are compared to that of the subsurface material. The present invention teaches the application of an ultrasound pulse only perpendicularly to a composite substrate. There is no teaching in Adler, as in the present invention, of the monitoring of a composite part being cured by using reflected ultrasound pulses to determine a change in the viscosity of the composite part.
U.S. Pat. No. 2,966,058 issued on Dec. 27, 1960 to H. J. McSkimin discloses a circuit for the measurement of dynamic properties of materials. The device includes a block of fused silica having a highly polished flat upper surface upon which is positioned a sample of test material. The apparatus then uses dual ultrasonic waves propagated through the silica substrate at different angles to determine various dynamic properties of the test material. This technique while using ultrasound waves does not show ultrasound pulses being directed normally to a curing composite part for the purpose of monitoring the viscosity of the composite part during the cure cycle.
U.S. Pat. No. 3,630,307 issued on Dec. 28, 1971 to Edwin C. Kamps et al for a mechanism and method for measuring sound absorption discloses an electromagnetic tone generator which transmits a tone burst comprising sound waves of known wavelength and intensity into one end of a tube. The other end of the tube is fitted against the surface of a sound attenuation material which reflects the sound waves back along the interior of the tube with a reduction in intensity proportional to the loss of acoustical energy. A microphone is exposed interiorly of the tube at a known distance from each end of the tube. Electronic time delay and gate circuitry feed into a monitoring circuit the electrical oscillations from the microphone produced by a selected series of clear sound waves of the original and the reflected sound bursts for a comparison of amplitudes. The Kamps patent is directed to the reflection of audible sound from a sound attenuation material for the purpose of determining its degradation during use. It is not directed to the use of ultrasound energy to monitor changes in the modulus of a composite part as it is being subjected to a cure cycle within an autoclave.
U.S. Pat. No. 4,510,103 issued Apr. 9, 1985 to Yasuhiro Yamaguchi et al relates to a method of molding a composite material containing a thermosetting resin by determining an optimum pressureinitiating time from abrupt variations in the electric capacity and/or dielectric loss coefficient of the molded material caused by the application of heat to the composite material. The purpose of this patent is only to determine the point at which to apply pressure to a mold rather than to continually monitor the viscosity of a curing composite part.
U.S. Pat. No. 4,145,912 issued Mar. 27, 1979 to Jacques L. P. Hognat relates to a method and apparatus and apparatus for measuring visco-elasticity of composite sheet material. This patent involves heating a sample of the sheet material comprising a flexible organic and/or inorganic reinforcement impregnated with a synthetic resin and stretching the sample while a reciprocal movement is applied to the sample. Variations in the amplitude of motion received at the other end are observed.